1. Field of the Invention
The invention relates to a contour collimator having a variable aperture.
2. Description of the Prior Art
Contour collimators having a variable aperture are used, e.g. in radiation therapy, because malignant tissue is destroyed by exposing it to a powerful gamma (photon) or electron radiation beam. At the same time, it is important that healthy tissue not be damaged. For this purpose, in radiation therapy equipment a contour collimator is placed between the source of radiation and the irradiated area, for limiting the radiation to essentially the malignant tissue.
A very common practice is to use contour collimators that consist of four aperture plates arranged at right angles to one another and which can be shifted with respect to one another. Thus, with collimators of this kind, only a rectangular radiation field can be produced. On the other hand, the malignant tissue usually has an irregular shape, so that healthy tissue may also be irradiated by the therapy.
In many oncological problem situations, better therapeutic results can be achieved if the distribution of the radiation dose is adapted to the, usually irregular, shape of the tumors. For this purpose, irregularly shaped auxiliary collimators are often constructed individually for the patients in radiation therapy. With the aid of patterns based on X-ray images, irregular field shapes are cut out of rigid foam boards, and these are cast with metal alloys having a low melting point. Thus, for every patient one or more cast collimators must be prepared. The disadvantages of these kinds of collimators are the expensive manufacturing process and the large amount of space required for storage.
A contour collimator of the above-mentioned kind, having an adjustable aperture, is known from EP-A-0 245 768. To make it adjustable, two stacks of aperture plates, which can be shifted with respect to one another, are mounted opposite one another. The lateral surfaces of the aperture plates in the stacks, which can be introduced into the aperture, are oriented toward the midpoint of the source of radiation. Collimators of this kind are also referred to as focused collimators. By this means, it is ensured that the radiation emitted from the midpoint of the radiation source always hits the same absorption cross-section. However, the construction of a focused collimator is expensive because the aperture plates must assume different angles, depending on how far they are inserted into the aperture. This requires an appropriate guide system for the aperture plates. In addition, further steps must be taken to prevent scatter radiation from reaching healthy tissue through the slits that are aligned with the source of radiation.